1. Field of the Invention
This invention relates to a method and apparatus for heat treatment of a preheated, largely calcined, fine grained product which consists of or contains lime, particularly raw cement meal. The meal is heated in a heating assembly from the temperature level of the decomposition of calcium carbonate (approximately 900.degree. C.) to the temperature at which alite formation begins (approximately 1250.degree. C.) and is then sintered into clinker in a burning assembly.
2. Description of the Prior Art
The dry burning method is used predominantly at present for the manufacture of cement, particularly portland cement, for reasons of heat efficiency. Originally, the entire process of preheating, deacidification and sintering were carried out in a rotary tubular kiln. Then, the step of preheating of the material was shifted to a heat exchanger outside the rotary tubular kiln. Finally, the deacidification step with its relatively high heat consumption was transferred to a separate calciner. These improvements have the advantage that due to better heat transmission with the fine grained product in suspension in hot gases, both the preheating as well as the calcination were significantly more efficient than in a product bed, as a result of which the specific heat consumption when burning cement could be considerably reduced.
Approximately 30 to 40% of the overall fuel charge is still used for the sintering burning in the rotary tubular kiln when there are separate heat treatment steps and complete deacidification in a calciner. The term "sintering burning" includes the heating of the product from 900.degree. C. following the calcination up to a temperature in the range where alite formation begins (approximately 1250.degree. C.) and the range of partial melting (above approximately 1300.degree. to 1340.degree. C.) as well as the dwell time of the material at this high temperature range up to a nearly complete bonding of the free CaO as alite.
Various investigations have come to the conclusion that it is highly desirable to quickly heat the calcined material to sintering temperature since, by so doing, the activity of the calcined material can be exploited with low heterogeneity for an accelerated lime bonding (i.e., alite formation). In standard cement burning installations, where not only the heating but also the sintering must be performed in the same rotary tubular kiln, the demand for fast heating by means of an increase of conveying speed in the kiln, however, can only be carried out very incompletely since the sintering requires exactly the opposite condition, namely, a dwell time for the material.
There have been a number of proposals which seek to realize a faster heating of the deacidified material to sintering temperature. Some solutions make use of a rapid heating which occurs outside of the rotary kiln (EP OS 52 429 and EP OS 52 431). In these proposals, the deacidified raw cement meal is placed in suspension with hot exhaust air from the clinker cooler and is heated from a temperature of 800.degree. through 850.degree. C. to a temperature of 1300.degree. through 1450.degree. C. with a single-shot addition of fuel before it is introduced into the rotary tubular kiln so that the burning product and the combustion exhaust gases are in concurrent flow. These proposals would seem to offer the possibility in addition to attaining a reaction kinetic advantage of adapting the rotary kiln to the demands of sintering, from which a cost saving, considerable shortening of the rotary tubular kiln would result.
More advanced proposals completely eliminate the rotary tubular kiln and transfer both the heating of the deacidified product to sintering temperature as well as the sintering itself into a suspension stream (German AS 25 50 384). Since only a very short time span is available for the sintering in such a method, a very high sintering temperature must be reached because a complete lime bonding would not be attainable even with a partially recirculating product. This presents a rather considerable problem in operation of such installations because more of the product becomes molten with increasing sintering temperatures, leading to cakings at the necessarily cooler reactor walls. This danger of reaching too high a sintering temperature and/or cakings of product components which have become molten also exists in a similar form for heating assemblies which heat the calcined burning product only up to the temperature ranges standard for sintering in the rotary tubular kiln with single addition of fuel and combustion air because topical overheatings cannot be excluded.